This chapter focuses on classic galactosemia, a potentially lethal inherited disorder caused by the deficiency of galactose-1-phosphate-uridyltransferase (GALT). The diagnosis of galactosemia is established by measurement of erythrocyte GALT enzyme activity and galactose-1-phosphate concentration, and GALT molecular genetic testing. Newborn screening utilizes a small amount of blood obtained from a heel prick to assay GALT enzyme activity and to quantify total RBC gal-1-P and galactose concentrations. In the absence of GALT activity, gal-1-P, galactose, and galactitol accumulate in a retrograde fashion and may be detected. Galactosemia is a disorder of galactose metabolism that can result in life-threatening complications including feeding problems, failure to thrive, hepatocellular damage, bleeding, and sepsis in untreated infants. Endogenous galactose production, metabolite depletion, and abnormal glycosylation of glycoproteins and glycolipids may account in part towards the long-term complications. A lactose-free diet will prevent and reverse metabolic crises or liver failure.